The present invention relates to a four-wheel drive motor vehicle of a type having a center differential between the front and rear wheels, and means for restraining or locking the center differential. More specifically, the present invention relates to an automatic control system for restraining or locking a center differential of a four-wheel drive system in accordance with a coefficient of friction between a road surface and a tire of the vehicle.
A four-wheel drive vehicle is superior in ability of hill climbing and ability of rough road driving. Even when a coefficient of friction between the tires and road surface is low, a four-wheel drive vehicle is very stable, as compared with a two-wheel drive vehicle, because the four wheels produce a frictional driving force, and accordingly the tires are prevented from sliding on the road surface. If, however, all four wheels are always driven at the same speed, a four-wheel drive vehicle cannot be turned smoothly. During a turn with large steering angles of steerable wheels, the front wheel of an inner or outer side must travel along a circle having a large radius and the rear wheel of the same side must travel along a small circle. Because of this difference in turning radius, there arises a large difference between a rotation speed (an average rotation speed, to be exact) of the front wheels and a rotation speed (an average rotation speed) of the rear wheels. As a result, the steering becomes heavy, the tendency to understeer is increased, and the vehicle cannot be turned without abnormal tire friction (called tight corner brake) which tends to brake the vehicle and cause an engine stall.
To overcome these handling and tire friction problems, some four-wheel drive systems use a center differential between front and rear wheels. If, however, one of the four wheels run on a muddy place, the center differential coacts with a rear or front differential between right and left wheels, so that the torque cannot be transmitted to the other three wheels, and the vehicle cannot escape from the muddy place.
In view of this problem, some four-wheel drive systems are further provided with means (a lockup mechanism or a non-slip differential mechanism) for restraining or preventing the action of the center differential. One example is shown in Japanese patent provisional publication No. 57-114727.
If, however, such a four-wheel drive vehicle with its center differential in the released state runs on a slippery road surface where the coefficient of friction is low, then the individual tires of the vehicle tend to slide separately on the slippery road surface through the action of the center differential, so that the vehicle stability during running and braking is reduced.